Support installing PHPUnit using composer.

"phpunit/phpunit" already exists inside our composer.json's "require-dev" however this has been
entirely useless as we don't include the autoloader which would load composer's PHPUnit.

This change begins including composer's autoloader when present and also tweaks phpunit.php
to ensure PHPUnit isn't double loaded. As a result besides supporting PHPUnit via composer this
also means that we're ready to make use of any library we add to our composer.json in the future.

Change-Id: I891740e8fd3d237c5f473862027205d951f564b9
Merge "Support installing PHPUnit using composer."